1. Field of the Invention
The present invention relates generally to the characterization of nanostructures on opaque or thick substrates. More particularly, the present invention relates to electron beam metrology in both reflection and backscattering modes applicable to the large sample sizes encountered in today's semiconductor fabrication environments.
2. Description of the Related Art
Reflection high-energy electron diffraction (RHEED) is a powerful analytical tool widely used for characterizing thin film growth in molecular beam epitaxy. It provides great sensitivity in measuring the atomic arrangements, which are in sub-nanometers, of a surface layer by monitoring the electron diffraction patterns; it also has been applied to measuring surface morphology by monitoring total reflection intensity [Ayahiko Ichimiya and Philip I. Cohen, Reflection High-Energy Electron Diffraction Cambridge University Press, 2004 (ISBN 0 521 45373 9)].
In a typical operation of RHEED, its incident electron beam often encompasses an energy range from about 8 to 20 kilo electron volt (KeV), though it can be employed at electron energies as high as 50 KeV to 100 KeV. To characterize atomic arrangements, the incident electron beam impinges the sample surface at a low glancing angle of a few degrees, and a diffraction beam spread over a range of few degrees. There are no lenses or electron optics between the sample and the detector since the angular range of the pertinent signals exists from a few degrees to tens of degrees. At a reasonable distance, the resolution of today's detector used in RHEED is sufficient to resolve the diffraction pattern in the reflected electron beam. However, it is not feasible to use the conventional RHEED technique to resolve the small-angle scattering signals to reach a sufficient angular resolution in characterizing structures on a nanometer scale.
Throughout this disclosure, the term “diffraction” is used in its classical definition adopted in X-ray crystallography; a probing beam, which can be X-ray, electron or neutron, after impinging on a crystalline material, will be diffracted if the condition specified by Bragg's law is fulfilled. Based on Bragg's law, the scattering or the diffraction beam occurs in wide angles from a few degrees to tens of degrees as the wavelength of the probing beam is comparable or larger than the characteristic length scale of the target material. In X-ray applications this phenomenon is dubbed as wide angle X-ray scattering, this name indicates that diffraction is also a scattering event caused by crystalline lattices and the diffraction angle is between a few degrees to tens of degree; hence the term “wide angle” in contrast to the term “small angle” appearing in the title of this application. Conversely, as the wavelength of the probing beam is less than the characteristic length of the target material, the scattering occurs in small angles of a few degrees or less.
Measurements similar to RHEED have also been conducted in transmission electron microscopy (TEM) and it has been coined as reflection electron microscopy (REM). This was first developed by Honjo and Yagi's group in exploring the atomic re-organization on silicon single crystal surface. [K. Yagi, K. Takayanagi, and G. Honjo, (1982), In Crystals, Growth, Properties and Applications vol. 7 Springer-Verlag, Berlin-Heidelberg, pp. 48-74] Using a high-energy electron beam to characterize structures of nanometer scale or at high angular resolutions has been conducted in conventional TEM; however, all the measurements have been carried out by monitoring the transmitted electron beam i.e. in transmission mode instead of the reflection and/or backscattering modes.
Transmission small-angle X-ray scattering (tSAXS) [T. Hu, R. L. Jones, W. L. Wu, E. K. Lin, Q. H. Lin, D. Keane, S. Weigand and J. Quintana, J. Appl. Phys. 96, (2004) pp. 1983-1987.] and grazing incident small-angle X-ray scattering (GISAXS) [J. Wernecke, M. Krumrey, A. Hoell, R. J. Kline, H. K. Liu and W. L. Wu, J. Applied Crystallography 47(6) (2014) pp. 1912-20] are two other relevant techniques using X-ray to probe nanostructures on flat substrates. The former one chooses X-ray with sufficient energy to penetrate the substrate, e.g. for silicon wafers commonly used in Today's semiconductor fabrication the incident X-ray used was typical above 13 KeV for a sufficient transmission power over ˜0.7 mm silicon wafer. Synchrotron X-ray sources have often been used for tSAXS measurement and this approach is not amendable to the use in testing laboratories or fabrication lines in semiconductor industries.
Current laboratory X-ray sources can provide appropriate energy level for tSAXS as well as a reasonably small beam size, about 50 μm, on samples; however, the measurement time is often in the range of hours or even longer due to the limited X-ray flux available from today's laboratory X-ray sources. To overcome this deficiency of low X-ray flux, GISAXS has been considered as a viable alternative. By lowering the incident angle from 90 degrees, as in the case of tSAXS, to a few degrees, the footprint, hence, the sampling area of GISAXS can be increased significantly. This, in turn, leads to an increase in scattering signal over tSAXS for a given incident X-ray flux. However, a large footprint of the incident beam on samples is impractical for many applications, e.g. the test pattern in semiconductor fabrication is often limited to 100 μm×100 μm or less.
The feature size in the nanostructures produced by today's semiconductor industries approaches 10 nanometers (nm) and beyond, which corresponds to an angle range of a few hundredths to a few thousandth of a degree when electron beams at kilo electron volts (KeV) are used as the probe. This minuscule angular range necessitates a novel high resolution apparatus operated in either a reflection or backscattering modes—the aim of this patent application.